In an active matrix type liquid crystal display apparatus, each thin-film transistor (TFT) is disposed to each of plural pixels disposed in a matrix form and the charge coming from and entering each pixel electrode is controlled by a switching function of TFT.
The fundamental constitution of the active matrix liquid crystal display apparatus is composed of two substrates facing each other, one of them is called a TFT substrate having a pixel region and the other is called a counter substrate. The TFT substrate is constituted by a pixel region containing several tens to several millions of pixel switching TFTs (hereinafter, is referred to as pixel TFTs) and a peripheral driving circuit region containing plural TFTs driving them.
An oriented film is formed on the TFT substrate and the counter substrate and an orientation treatment such as rubbing, etc., for arranging the orienting property of the liquid crystal material is carried out. It is desirable that the surfaces of the TFT substrate and the counter substrate directly before forming the oriented film thereon have planes as flat as possible because the surface states relates to the orientation of a liquid crystal.
Thereafter, to keep the distance between the TFT substrate and the counter substrate, spherical spacers are uniformly scattered on one of the TFT substrate and the counter substrate. Then, the two substrates are adhered to each other by a sealing material, after dividing the assembly, a liquid crystal material is filled between the TFT substrate and the counter substrate, and the liquid crystal inlet is sealed by a sealing material.
As the main heat treatments carried out in the above-described processes, there are;
burning of the oriented films (after keeping for one hour at 180.degree. C., the film is gradually cooled to normal temperature), PA1 the heat press at adhering the substrates (after keeping for 3 hours at 160.degree. C., the temperature is gradually lowered to 100.degree. C. over a period of 2 hours, and then the temperature is gradually lowered to normal temperature over a period of 2 hours), and PA1 Burning for making a re-orientation treatment (after keeping for 30 minutes at 120.degree. C., the film is gradually cooled to normal temperature). PA1 a step of forming plural electrodes on an interlayer dielectric, PA1 a step of heat-treating the plural electrodes to form protrusions on the surfaces of the plural electrodes, and PA1 a step of removing the protrusions on the surfaces of the electrodes to flatten the surfaces. PA1 a step of forming plural pixel electrodes on an interlayer dielectric, PA1 a step of heat-treating the plural pixel electrodes to form protrusions on the surfaces of the plural pixel electrodes, and PA1 a step of removing the protrusions on the surfaces of the plural pixel electrodes to flatten the surfaces. PA1 a step of forming plural electrodes on a 1st interlayer dielectric, PA1 a step of heat-treating the plural electrodes to form protrusions on the surfaces of the plural electrodes, PA1 a step of forming a 2nd interlayer dielectric covering the plural electrodes, and PA1 a step of flattening the surfaces of the plural electrodes and the surface of the 2nd interlayer dielectric such that both the surfaces are in a same plane. PA1 a step of forming plural electrodes on a 1st interlayer dielectric, PA1 a step of forming a 2nd interlayer dielectric covering the plural electrodes, PA1 a step of flattening the surfaces of the plural electrodes and the surface of the 2nd interlayer dielectric such that both the surfaces are in a same plane and filling boundary portions or gaps among the plural electrodes with the 2nd dielectric film, PA1 a step of heat-treating the plural electrodes to form protrusions on the surfaces of the plural electrodes, and PA1 a step of removing the protrusions of the surfaces of the plural electrodes to flatten the surfaces.
In the case of making a reflection type LCD wherein a metal material is used as the pixel electrode, the light reflectance of the surfaces of the electrodes is lowered by the above-described heat treatments.
In the reflection type LCD, there are various display modes of a liquid crystal and, for example, there is a display mode of attaching importance to reflect the polarized component of light at the pixel electrodes as an ECB (electric field control birefringence) mode.
Accordingly, even when lowering of the reflectance of light is slight, it gives an influence on the polarized component of light and further it gives a bad influence on the display.
As described above, because in the reflection type LCD, the reflectance of the pixel electrodes is important, it has become a problem that the reflectance is lowered by the heat treatments.
The cause of lowering the light reflectance at the surfaces of the electrodes is explained below.
That is, when the pixel electrodes are formed with aluminum which is a material having a high reflectance and a low resistance, by the influence of the heat treatment after forming the pixel electrodes or the heat treatment in a cell-assembling process, hillocks or whiskers forms on the surfaces of the electrodes caused by the abnormal growth of aluminum. This is the main cause of lowering the reflectance of light by the heat treatment.
Now, the term "hillocks" is a phenomenon that swells form by colliding with each other of the growing components of aluminum. Also, the term "whiskers" is a phenomenon that a splinter-like growth occurs by the abnormal growth of aluminum.
The grown length of these hillocks and whiskers reaches several .mu.m.
Thus, an experiment for determining the change of the reflectance of an aluminum alloy by a heat treatment (heat-treatment=one hour) was carried out. In the experiment, a film of Al--Ti(1%) formed at room temperature using a sputtering method was used. In the conditions at the film formation, an electric power was 3000 W and "T-S" was 150 mm at 0.4 Pa, wherein said "T-S" means a distance between a target and a sample.
Averages of the reflectances in the wavelength region of from 400 nm to 800 nm obtained in the experiment is shown in FIG. 10 as the light reflectance-baking temperature reliance of the film of Al--Ti(1%).
It can be seen from FIG. 10 that at the film formation (room temperature) of the film of Al--Ti(1%), the film has the light reflectance of from about 88 to 90% but because hillocks and whiskers form on the surfaces of the electrodes with the increase of the heat-treatment temperature (baking temperature), the light reflectance is lowered.
The light reflectance of aluminum in the visible light region is very high as compared with other metals and alloys (the reflectance of pure aluminum is from 92 to 93%), and is suitable for use as the pixel electrodes of the reflection type LCD.
In pure aluminum, hillocks are formed by a heat treatment of 100.degree. C. or higher. Accordingly, hitherto, at least 0.5%, and preferably at least 2% titanium, scandium, or silicon is added to aluminum and the aluminum alloy is used as a material restraining the formation of hillocks.
As described above, when aluminum contains Ti, etc., hillocks are hard to form but on the other hand, the reflectance of the aluminum alloy is lowered by several percents as compared with pure aluminum as well as there occurs a problem that the resistance of such an aluminum alloy is increased.
Thus, hitherto, by the heat treatment processes (protective film formation, cell assembling, etc.) after the film formation of pixel electrodes composed of aluminum, hillocks form on the surface of the pixel electrodes and the light reflectance is lowered.
As described above, hitherto, in the case of using aluminum for pixel electrodes, hillock and whiskers are formed on the light-reflecting surfaces of the pixel electrodes to cause a problem of lowering the light reflectance.